How do I make those rhombic indents in this water bottle? Image in description
To make the shape, use the Mirror, Array and Simple Deform (Bend mode) modifiers:
Create a plane, rotate it, apply the rotation, subdivie, mirror:

Array it 6 times, make it bend with a Simple Deform modifier (Bend mode)

Change its shape, extrude inwards:


Give it a Subdivision Surface modifier, add edge loops to sharp the edges:

As the medium part is divided by 4 and not 6, you're going to make it apart. Draw the curve with some extrusion, delete the faces to only keep the edges:

Create another plane, subdivide it so that it has the same number of edge loops as the bottom part (18 faces), use the previous shape to cut it with the Knife Project tool:

Now again, mirror, array and bend with a Simple Deform modifier:

Extrude inwards, add edge loops:

Merge with the bottom:

Correct the shape etc:

You probably need half the edges, you can simplify whenever you want: